a. Field of the Invention
The present invention refers to a down-the-hole rock drill.
b. Description of Related Art
The most common way by far to provide rotational driving between the shaft of a drill bit and a driver sub is to use splines both on the exterior of the shaft and on the wall of the bore of the driver sub. However, such a design of the cooperating rotational driving means of the shaft of the drill bit and the bore of the driver sub is complicated to manufacture and consequently the manufacturing is time consuming and expensive. According to GB-B-1 242 052 the conventional splines design suffers from the disadvantage that the splines of the chuck (driver sub) and the drill bit are caused to wear very rapidly by the repeated relative axial and angular movements between them.
From GB-B-1 242 052 a percussive rock drilling apparatus of the type defined above in the first paragraph is previously known. The aim of the design of the rotational driving means between the driver sub and the shaft of the drill bit as disclosed in GB-B-1 242 052 is to avoid said disadvantages concerning rapid wear and angular movements.
The known device according to GB-B-1 242 052 has a transverse pin secured in the driver sub (chuck) of the device, said pin cooperating with an axially extending flat surface on the shaft of the drill bit to restrict the axial displacement of the drill bit. However, such a structural design of the means for restriction of axial displacement of the drill bit relative to the driver sub will negatively affect the longitudinal centering of the shaft, during working conditions, relative to the bore of the driver sub. This is an extremely important feature for drill bits having an internal bore extending axially through the drill bit, i.e. drill bits of the type used in the down-the-hole rock drill according to the present invention. If, during working conditions, the shaft of the drill bit is not properly centered, relative to the bore of the driver sub, then there will be problems in the cooperation between the piston and the drill bit as regards the foot valve and the energy transmission.
Since the cooperation of the rotational driving means of the driver sub and the drill bit of GB-B-1 242 052 do not provide a longitudinal centering of the drill bit relative to the bore of the driver sub it is necessary to provide additional guiding means for the driver sub at the transition between the shaft and the head of the drill bit. The provision of such additional guiding means will of course negatively affect the manufacturing costs of the drill bit compared to a drill bit having no such guiding means.